Methods for preparing alkali cellulose and cellulose ether

ABSTRACT

The invention is a method for efficiently preparing alkali cellulose having a uniform alkali distribution therein. More specifically, the invention is a method for preparing alkali cellulose comprising steps of bringing pulp into continuous contact with an alkali metal hydroxide solution in a pipe type contactor to generate a contact mixture and draining the contact mixture; a method for preparing cellulose ether comprising use of the alkali cellulose; and an apparatus for preparing alkali cellulose, comprising a pipe type contactor having at least one inlet port at one end thereof for introducing pulp and an alkali metal hydroxide solution, and an outlet port at the other end for discharging a contact mixture, wherein the pulp and the alkali metal hydroxide solution are moved from one end to the other end while bringing them into contact with each other, and a drainer for separating a cake from the contact mixture discharged from the contactor.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to alkali cellulose and a method forpreparing cellulose ether using the same.

2. Description of the Related Art

For preparation of cellulose ether, known is a method comprising stepsof bringing highly purified pulp into contact with an alkali solution toprepare alkali cellulose, and then etherifying the alkali cellulose withan etherifying agent. Although the cellulose ether obtained as a finalproduct becomes soluble in water by properly controlling the degree ofsubstitution, the cellulose ether contains a water-insoluble portion sothat the light transmittance of the aqueous solution may be lowered orthe commodity value of the cellulose ether may be damaged as a foreignmatter.

The insoluble portion is caused by the presence of a portion having alow degree of substitution which does not have enough substituents topermit dissolution of the cellulose ether in water. One of the causes ofit is uneven alkali distribution in the alkali cellulose.

The rolls the alkali plays includes (a) swelling the cellulose to changethe crystal structure in the pulp, thereby accelerating the penetrationof the etherifying agent, (b) catalyzing the etherification reaction ofan alkylene oxide; and (c) serving as a reactant for an alkyl halide. Aportion of the pulp out of contact with the aqueous alkali solution isnot involved in the reaction and therefore remains as an insolubleportion. Thus, uniformity of the alkali cellulose is related to anamount of the insoluble portion in the alkali cellulose.

A method which has been employed widely for the preparation of an alkalicellulose includes one described in described in Japanese PatentApplication Examined Publication No. 60-50801/1985 or Japanese PatentApplication Unexamined Publication No. 56-2302/1981 in which an alkaliis added in an amount necessary for etherification to pulp powderobtained by pulverization of pulp, and then mixed by mechanically. Inthis method, however, the alkali is not distributed to all parts of thepup powder so that some parts of the pulp powder remain out of contactwith the alkali. As a result, some parts fail to become a celluloseether, stay in the product as an unreacted substance and deteriorate thequality of the cellulose ether. Thus, this method causes a problem.

In order to avoid causing such a problem, a method comprising steps ofimpregnating a sheet of pulp in an excess alkali solution to allow thepulp to absorb a sufficient amount of an alkali and then pressing theimpregnated sheet to remove an unnecessary portion of the alkali andcontrol the alkali to a predetermined amount. When this method isperformed industrially, it is the common practice that a rolled pulp isallowed to be rotated freely by elevating the rolled pulp by asupporting shaft put through the central pipe of the roll so as to bringit up from a floor surface, or placing the rolled pulp over a roller.Then, a sheet is drawn from the rolled pulp and introduced into animmersion tank. However, according to this method, the operation isoften interrupted because the pulp sheet is torn by a pull force duringthe immersion. Moreover, to achieve mass production, a huge immersiontank is required for immersing the pulp sheet therein for apredetermined time. Accordingly, this method has defects such asnecessity of an adequate space for the tank and inevitable rise in theinvestment cost. On the other hand, when pulp chips are employed, thecake of them has an irregular surface so that the uneven squeeze occurswhen pressed by the press machine. Uneven alkali distribution caused bythe uneven squeeze deteriorates the quality of the alkali cellulose.

In page 433 of “Encyclopedia of Cellulose” edited by The CelluloseSociety of Japan (published on Nov. 10, 2000), described is apreparation method of alkali cellulose for preparation of viscose, thestep comprising adding pulp to an alkali solution to form a gruel-likeslurry and squeezing the slurry by a slurry press. Although the defectscaused by use of a sheet of pulp are solved, the slurry press causesuneven squeeze and thereby deteriorates the quality of the alkalicellulose due to uneven alkali distribution. In addition, it isdifficult to obtain alkali cellulose having a relatively small alkalicontent which is required as a raw material of cellulose ether usingthis method alone because of the limitation of the squeeze performance.Thus, it is difficult to apply this method to the preparation ofcellulose ether.

In Japanese Patent Publication Examined Publication No. 3-73562/1991,described is a method of preparing alkali cellulose having a desiredcomposition, comprising steps of preparing alkali cellulose fromcellulose and excess alkali, and then washing the alkali cellulose witha hydrophilic solvent for removal of the alkali. However, this methodneeds huge equipment and many operations. In addition, the hydrophilicsolvent remains in the alkali cellulose and causes a side reaction withan etherifying agent. Accordingly, a reaction efficiency of theetherifying agent is reduced. Neutralization of the washing liquid orrecovery of the alkali is required. Thus, this method is industriallydifficult.

SUMMARY OF THE INVENTION

The present invention provides a method for efficiently preparing alkalicellulose having a uniform alkali distribution.

In an aspect of the present invention, there is thus provided a methodfor preparing alkali cellulose, comprising steps of:

bringing pulp into continuous contact with an alkali metal hydroxidesolution in a pipe type contactor to generate a contact mixture and

draining the contact mixture.

In another aspect of the present invention, there is also provided amethod for preparing cellulose ether, comprising use of alkali cellulosethus prepared.

In a further aspect of the present invention, there is also provided anapparatus for preparing alkali cellulose, comprising:

a pipe type contactor comprising

-   -   at least one inlet port at one end for introducing pulp and an        alkali metal hydroxide solution and    -   at least one outlet port at the other end for discharging a        contact mixture,    -   wherein the pulp and the alkali metal hydroxide solution can be        moved from one end to the other end while bringing them into        contact with each other to generate the contact mixture; and

a drainer for separating a cake from the contact mixture discharged fromthe outlet port.

According to the present invention, alkali cellulose having a uniformalkali distribution therein can be prepared efficiently. As a result,cellulose ether having high transparency can be prepared efficiently.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 illustrates an example of a preparation apparatus of alkalicellulose.

FIG. 2 illustrates an example of a preparation apparatus having a rotaryvalve connected to a pulp inlet.

FIG. 3 illustrates an example of a preparation apparatus to which asnake pump equipped with a hopper has been connected.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The pulp to be used in the present invention may be preferably in theform of powder or chips.

The pulp powder may be available by pulverizing a pulp sheet and it isin the form of powder. An average particle size of the pulp powder maybe, but not limited to, usually 10 to 1,000 μm. Although no limitationis imposed on the preparation method of the pulp powder, a pulverizersuch as knife and hammer mill can be used.

Although no limitation is imposed on the preparation method of pulpchips, the pulp chips may be produced by cutting a pulp sheet with aknown cutting device such as a slitter cutter. A continuous cuttingdevice may be advantageous from the viewpoint of investment cost. Theplane surface area of the chip may be usually from 4 to 10,000 mm²,especially preferably from 10 to 2,500 mm². It may be difficult toprepare the chip having the plane surface area smaller than 4 mm². Thechip having the plane surface area greater than 10,000 mm² may posedifficulties in handling such as introducing into a pipe type contactor,transferring inside of the contactor, and introducing into a drainer.Considering the pulp chip as a hexahedron, the plane surface area of thechip is the largest surface area of six surface areas of the hexahedralchip.

In the present invention, a ratio of the weight of the pulp to thevolume of the alkali metal hydroxide solution, which flow through a pipeper hour, may be preferably 0.10 kg/L or less, more preferably 0.05 kg/Lor less, still more preferably 0.02 kg/L or less. When the ratio exceeds0.10 kg/L, clogging may tend to be caused inside the pipe. The lowerlimit of this ratio may be preferably 0.0001 kg/L. If this lower limitis not satisfied, the equipment may become impractically large.

The pipe type contactor to be used in the present invention may be along tubular container and an alkali metal hydroxide solution flowsinside the pipe type contactor.

The length of the pipe can be changed as needed. For example, the lengthof pipe may be changed by providing a plurality of pipes different inlength and switching the flow path from one pipe to the other. Thelength of pipe may be also changed by providing some extraction valvesin the pipe and changing the extracting position. A double pipe may beoptionally employed so that a heat medium for temperature control canflow in the outer tube thereof.

An inner diameter of the pipe may be preferably greater, especiallypreferably at least three times greater than the size (the longest side)of pulp chips in order to prevent clogging therewith.

FIG. 1 illustrates an example of a preparation apparatus of alkalicellulose comprising a pipe type contactor 10 and a drainer 20 fordraining a contact mixture generated in the pipe type contactor toseparate a cake therefrom.

A premixture of an alkali metal hydroxide solution 2 from an alkalimetal hydroxide solution tank 4 and pulp 1 from a pulp feeder 5 isintroduced by a pump 6 into an inlet port 11 at one end of the pipe typecontactor 10. Alternatively, the alkali metal hydroxide solution andpulp may be introduced into respective inlet ports without premixingthem.

The pulp and the alkali metal hydroxide solution thus introduced may betransported from the one end to the other end of the contactor whilebringing them into contact with each other. Subsequently, the contactmixture may be discharged from an outlet port 12. The contact mixturethus discharged may be separated by the drainer 20 to obtain alkalicellulose 3 in the form of a cake. As a driving unit for causing thecontact mixture to flow through the pipe, a known unit such as a drivingpump may be used. The liquid recovered by the drainer may be sent to thealkali metal hydroxide solution tank 4 by a pump 21 and can be recycled.

The pulp may be introduced into the pipe type contactor by, asillustrated in FIG. 2, connecting a rotary valve 7 to the pulp feeder 5and introducing the pulp via the valve.

The pulp may also be introduced into the pipe type contactor by, asillustrated in FIG. 3, introducing a premixture, which have beenprepared using a snake pump 8 equipped with a hopper, into the inletport 11. Alternatively, a whopper may be placed at a suction part of aslurry pump so as to receive pulp and an alkali metal hydroxidesolution.

When the pipe has a downward slope, an interface appears between aliquid phase and a gas phase inside the pipe so that the cross-sectionalarea of the liquid phase may not coincide with the cross-sectional areaof the pipe, which may pose a problem in the control of a contact time.Accordingly, it may be preferable to increase a portion of an upwardslope.

It may be preferable that the pipe type contactor can freely control atemperature of the alkali metal hydroxide solution or a contact time.The composition of the alkali cellulose varies depending on theabsorption rate of the alkali metal hydroxide solution by the pulp andthis absorption rate can be adjusted by controlling the temperature ofthe alkali metal hydroxide solution or the contact time. Accordingly,the apparatus capable of controlling it can produce alkali cellulosehaving a desired composition.

Although the temperature of the alkali metal hydroxide solution can becontrolled by a known technique, use of a heat exchanger is preferred.The heat exchanger may be installed inside or outside the pipe. Althoughno particular limitation is imposed on the temperature of the alkalimetal hydroxide solution, it may be adjusted to be within a range offrom 20 to 80° C. The pipe contactor capable of carrying out continuoustreatment may be preferred. A continuous apparatus can be made smallerin size than a batch type apparatus so that it is advantageous in viewof space economy.

The contact time can be adjusted preferably by changing the flow rate ofthe alkali metal hydroxide solution and/or the length of a contact zone.

The flow speed in the pipe is determined by the flow rate of the alkalimetal hydroxide solution. The linear speed in the pipe may be preferably0.1 m/s or greater, especially preferably from 0.2 to 10 m/s. When it isless than 0.1 m/s, the movement of pulp in the pipe may becomedifficult. When it is more than 10 m/s, a pressure loss may becomeexcessively large, disturbing smooth operation. The flow rate can bechanged by an ordinarily method, for example, including change of arotational speed of a slurry pump or use of a control valve.

The length of the contact zone in the pipe may be preferably 0.1 to 200m, especially preferably 0.2 to 100 m. When it is less than 0.1 m, itmay be difficult to control the amount of absorption. When it exceeds200 m, the equipment may become impractically large.

The contact time between the pulp and the alkali metal hydroxidesolution may be preferably from 1 second to 15 minutes, especiallypreferably from 2 seconds to 2 minutes. When the contact time is lessthan 1 second, it may be highly difficult to control the absorptionamount. When the contact time is more than 15 minutes, the apparatus maybe excessively large so that productivity may be deteriorated. Inaddition, the pulp may absorb too much amount of the alkali so that itmay become difficult for any drainer to prepare alkali cellulose havinga desired composition suited for the preparation of cellulose ether.

It may be preferable that the pipe type contactor has a main bodypermitting water circulation by a jacket so that the heat generated bymixing of the alkali and cellulose can be controlled and thereby theinternal temperature can be controlled.

The apparatus for bringing the pulp into contact with the alkali metalhydroxide solution can preferably treat them continuously. Such acontinuous apparatus is advantageous over a batch one in view of spaceeconomy because the apparatus itself can be made smaller than the batchone. The contactor preferably enables piston-flow passage of the pulp inview of good quality of alkali cellulose. The composition of alkalicellulose varies depending on the contact time as mentioned above sothat the constant contact time allows the alkali cellulose to have auniform composition. It should be avoided for the pulp introduced intothe contactor to pass through without contact with the alkali in orderto obtain a good quality product. Since the pulp is apt to float in thealkali metal hydroxide solution, the pulp has to pass through thecontactor while ensuring complete contact with the alkali metalhydroxide solution.

It may be preferable that the pipe type contactor can be evacuated ornitrogen-purged in order to prevent a reduction in the polymerizationdegree of the alkali cellulose in the presence of oxygen. If the controlof the polymerization degree in the presence of oxygen is desired at thesame time, the contactor having a structure capable of controlling anoxygen amount may be preferred.

No particular limitation is imposed on the alkali metal hydroxidesolution to be used in the invention insofar as alkali cellulose can beobtained using it. An aqueous solution of sodium hydroxide solution orpotassium hydroxide may be preferred, with the former one beingespecially preferred from an economical viewpoint. The concentration ofthe solution may be preferably 23 to 60% by weight, especiallypreferably 35 to 55% by weight. When the concentration is less than 23%by weight, it may be economically disadvantageous because a sidereaction between an etherifying agent and water may occur during thesubsequent preparation of cellulose ether. Moreover, cellulose etherhaving a desired degree of substitution may not be obtained and anaqueous solution of the cellulose ether thus prepared may not beinferior in transparency. When the concentration is more than 60% byweight, the solution may not be handled easily because of an increase inviscosity. The concentration of the alkali metal hydroxide solution tobe provided for the contact with the pulp may be preferably keptconstant in order to stabilize the composition of the alkali celluloseand ensure the transparency of the cellulose ether.

According to the invention, the uniformity of the alkali distributioncan be improved even if an inert solvent such as lower alcoholpreferably having 1 to 4 carbons is not used. However, use of such asolvent does not pose any problem. Use of such a solvent can improveuniformity of alkali distribution and bulk density of alkali cellulose.

According to the present invention, after the pulp and the alkali metalhydroxide solution are brought into contact with each other in the pipetype contactor, an excess alkali metal hydroxide solution is removedfrom the contact mixture by a drainer such as a squeezer so as to obtainalkali cellulose.

The drainer may include a drainer making use of a centrifugal force suchas a decanter or a rotating basket; a mechanical drainer such as a rolltype, a V-shaped disc press or screw press; and a vacuum filter. Fromthe viewpoint of uniform draining, a drainer making use of a centrifugalforce may be preferred. In addition, it may be preferable that a drainercan conduct continuous treatment. Examples may include a screw dischargetype centrifugal dehydrator, an extrusion plate type centrifugalseparator and a decanter. In a drainer making use of centrifugal force,a required draining degree can be attained by adjusting the rotationspeed of the drainer. In a mechanical drainer and a vacuum filter, arequired draining degree can be attained by adjusting a drainingpressure and a degree of vacuum, respectively.

The alkali solution recovered by draining can be recycled. When it isrecycled, an alkali metal hydroxide solution may be preferably suppliedto the system continuously in an amount equal to that of the alkalimetal hydroxide solution taken out of the system as alkali cellulose. Inthis case, it may be possible to transfer the alkali solution recoveredby draining to the tank once, and then feed it from the tank to anapparatus for contact, and then add a new alkali metal hydroxidesolution so as to keep a constant level in the tank.

When the alkali solution recovered by draining is recycled, it may beespecially preferable that both of a continuous centrifugal separatorhaving a perforated rotor and a continuous centrifugal separator havingan imperforate rotor are used together. This makes it possible toprevent the clogging of the centrifugal separator having a perforatedrotor and prevent filtration failure or oscillation of the centrifugalseparator caused thereby. First of all, a mixture obtained by thecontact of the pulp with the alkali metal hydroxide solution isseparated into a liquid and a solid by using a continuous centrifugalseparator equipped with a perforated rotor. Then, the fine solid in theseparated liquid is further separated by a continuous centrifugalseparator having an imperforate rotor. A part or all of the liquidseparated by the continuous centrifugal separator having a perforatedrotor can be directly introduced into the continuous centrifugalseparator having an imperforate rotor. Alternatively, it can be placedin the tank and then introduced into the continuous centrifugalseparator having an imperforate rotor. The solid recovered from theseparated liquid by the continuous centrifugal separator having animperforate rotor can be used as alkali cellulose.

The continuous centrifugal separator can control the number of rotation,depending on the contact time and the contact temperature between pulpand an alkali metal hydroxide solution, and necessary draining degree.In other words, the continuous centrifugal separator can control acentrifugal effect. The control of the centrifugal effect can keep theconcentration of the alkali metal hydroxide solution constant, thesolution being fed repeatedly for the contact with the pulp.Consequently, the cellulose ether having high transparency can beobtained. When the contact time and/or the contact temperature isincreased from the present operation, the centrifugal effect can bereduced. When the contact time and/or contact temperature is lowered,the centrifugal effect can be increased. It may be preferable to keepthe variation of the concentrations of the alkali metal hydroxidesolution within ±10%, especially preferably ±5%.

Incidentally, the centrifugal effect is a value indicating a magnitudeof the centrifugal force and is given by a ratio of the centrifugalforce to the gravity (see “New Edition Chemical Engineering DictionaryEdited by Society for Chemical Engineers, Japan”, published on May 30,1974). The centrifugal effect Z is represented by the followingequation.Z=(ω² r)/g=V ²/(gr)=n ² N ² r/(900 g)wherein “r” represents a rotational diameter (unit: m) of a rotator, “ω”represents an angular rate (unit: rad/sec) of a rotator, “V” representsa circumferential rate (unit: m/sec) of a rotator, “N” represents arotational number (unit: rpm) of a rotator and “g” represents agravitational acceleration (unit: m/sec²).

A weight ratio of the alkali metal hydroxide contained by the cakeobtained by draining to the solid portion contained by the pulp (alkalimetal hydroxide/solid portion in the pulp) may be preferably 0.3 to 1.5,more preferably 0.65 to 1.30, still more preferably 0.90 to 1.30. Whenthe weight ratio falls within a range of 0.3 to 1.5, the cellulose etherthus obtained has improved transparency. The solid portion in the pulpmay include, in addition to cellulose which is a main component,hemicellulose, lignin, organic matters such as a resin, and inorganicmatters such as Se and Fe components.

The weight ratio of (alkali metal hydroxide)/(solid portion in the pulp)can be determined by the following titration method.

First, 4.00 g of a cake is sampled and the weight percentage (wt %) ofthe alkali metal hydroxide contained in the cake is determined byneutralization titration (0.5 mol/L H₂SO₄, indicator: phenolphthalein).A blank test is also performed in a similar manner.Wt  %  of  alkali  metal  hydroxide = (normality  factor) × {(dropped  amount  (ml)  of  H₂SO₄) − (dropped  amout  (ml)  of  H₂SO₄  in  blank  test)}

Using the wt % of the alkali metal hydroxide contained by the cake, aweight ratio of the alkali metal hydroxide to the solid portioncontained by the pulp is then determined in accordance with thefollowing equation:(weight  of  alkali  metal  hydroxide)/(weight  of  solid  portion  in  pulp) = (wt  %  of  alkali  metal  hydroxide) ÷ [  {100 − (wt  %  of  alkali  metal  hydroxide)/(B/100)} × (S/100)].

In the above equation, “B” represents the concentration (wt %) of thealkali metal hydroxide solution and “S” represents the concentration (wt%) of the solid portion in the pulp. The concentration of the solidportion in the pulp is obtained by dividing the residual weight afterdrying about 2 g of sampled pulp at 105° C. for 2 hours, by the weightof the sampled pulp and is expressed by weight percentage.

A feed rate of the pulp to the pipe of the pipe type contactor; and arecovery rate of the alkali cellulose after draining, or a consumptionrate of the alkali metal hydroxide solution are measured. Thecomposition of the alkali cellulose at present is calculated based ontheir weight ratios. The contact time, a temperature of the alkali metalhydroxide solution in the pipe, or draining degree such as a squeezingpressure can be controlled so as to conform the calculated compositionto a target composition. The above-described measurements, calculationand controlling operations can be automated.

The composition of the alkali cellulose can be determined by theetherification degree of cellulose ether obtained from the alkalicellulose, that is, the molar substitution degree or value.

Using the alkali cellulose obtained by the above-described preparationmethod as a raw material, cellulose ether can be prepared in a knownmanner.

The reaction method may include a batch type or a continuous type. Thecontinuous type is employed for the preparation of the alkali celluloseaccording to the present invention so that continuous type for thepreparation of cellulose ether is preferred, but batch reaction isfeasible.

In the batch type, the alkali cellulose discharged from the drainer maybe stored in a buffer tank or charged directly in an etherificationreactor. It may be preferable to store the alkali cellulose in thebuffer tank and then charge the alkali cellulose in a reaction vessel ina short period of time in order to reduce its occupancy time in theetherification reactor. This leads to an increase in productivity. Thebuffer tank is preferably evacuated or purged with nitrogen so as toform an oxygen-free atmosphere therein, whereby a reduction in thepolymerization degree can be suppressed.

Examples of cellulose ether obtainable from the resulting alkalicellulose as a starting material may include alkyl cellulose,hydroxyalkyl cellulose, hydroxyalkylalkyl cellulose and carboxymethylcellulose.

Examples of the alkyl cellulose may include methyl cellulose having amethoxyl group of 1.0 to 2.2 (D.S.) and ethyl cellulose having ethoxylgroup of 2.0 to 2.6 (D.S.). It should be noted that D.S. (degree ofsubstitution) means the average number of substituted hydroxyl groups inthe anhydrous glucose unit, while M.S. (molar substitution) means theaverage number of substituents in the anhydrous glucose unit.

Examples of the hydroxyalkyl cellulose may include hydroxyethylcellulose having a hydroxyethoxyl group of 0.05 to 3.0 (M.S.) andhydroxypropyl cellulose having a hydroxypropoxyl group of 0.05 to 3.3(M.S.).

Examples of the hydroxyalkylalkyl cellulose may includehydroxyethylmethyl cellulose having a methoxyl group of 1.0 to 2.2(D.S.) and a hydroxyethoxyl group of 0.1 to 0.6 (M.S.),hydroxypropylmethyl cellulose having a methoxyl group of 1.0 to 2.2(D.S.) and a hydroxypropoxyl group of 0.1 to 0.6 (M.S.), andhydroxyethylethyl cellulose having an ethoxyl group of 1.0 to 2.2 (D.S.)and a hydroxyethoxyl group of 0.1 to 0.6 (M.S.).

Carboxymethyl cellulose having a carboxymethoxyl group of 0.2 to 2.2(D.S.) can also be given as an example of cellulose ether.

Examples of the etherifying agent may include alkyl halide such asmethyl chloride and ethyl chloride; alkylene oxide such as ethyleneoxide and propylene oxide; and monochloroacetic acid.

EXAMPLES

The present invention will hereinafter be described by Examples. Itshould not be construed that the present invention is not limited to orby these Examples.

Example 1

A pipe having an inner diameter of 38 mm and a length of 10 m was placedso as to have an upward slope. A snake pump equipped with a hopper wasconnected to the inlet of the pipe. In the hopper of the snake pump, a49 wt % aqueous sodium hydroxide solution of 40° C. was supplied at arate of 1,300 L/hr. At the same time, 4-mm square pulp chips produced bywood and having a solid concentration of 93 wt % were introduced at arate of 50 kg/hr. The rotational speed of the snake pump was regulatedso that the liquid level inside the hopper became a predetermined level.The linear speed in the pipe was 0.32 m/s, moving speed of the pulp was0.32 m/s, and retention time was 31 seconds.

The outlet of the pipe was connected to a V-shaped disc press and amixture of the pipe chips and the sodium hydroxide solution dischargedfrom the pipe was drained continuously. The weight ratio of the alkalimetal hydroxide contained by the alkali cellulose thus obtained to thesolid portion contained by the pulp, which was determined by thetitration method, was 1.25.

Example 2

In a similar manner to Example 1 except that a 49 wt % aqueous sodiumhydroxide solution was fed at a rate of 1,800 L/hr, and a screwdischarge type centrifugal dehydrator having a centrifugal effect of 600was disposed at the outlet of the pipe as a drainer, alkali cellulosewas prepared. The linear speed in the pipe was 0.44 m/s, moving speed ofthe pulp was 0.44 m/s, and retention time was 23 seconds. The weightratio of the alkali metal hydroxide contained by the alkali cellulosethus obtained to the solid potion contained by the pulp, which wasdetermined by the titration method, was 1.00.

Example 3

In a similar manner to Example 1 except that a pipe having a length of7.4 m was employed, and an extrusion plate type centrifugal dehydratorhaving a centrifugal effect of 600 and equipped with a 0.2 mm slitscreen was installed as a drainer at the outlet of the pipe, alkalicellulose was prepared. The weight ratio of the alkali metal hydroxidecontained by the alkali cellulose thus obtained to the solid portioncontained by the pulp, which was determined by the titration method, wasin 1.00.

Example 4

In a pressure-resistant reactor was charged 20 kg of the alkalicellulose obtained in Example 1. After vacuuming, 11 kg of methylchloride and 2.7 kg of propylene oxide were added for a reaction. Thereaction product was washed, dried and pulverized to yieldhydroxypropylmethyl cellulose.

The hydroxypropylmethyl cellulose thus obtained had a methoxyl group(D.S.) of 1.90 and a hydroxyl group (MS) of 0.24. A 2 wt % aqueoussolution of the hydroxypropylmethyl cellulose had a viscosity at 20° C.of 10,000 mPa·s. The light transmittance at 20° C. of a 2 wt % aqueoussolution thereof was 98.0% as a result of measurement using aphotoelectric calorimeter “PC-50”, a cell length of 20 mm and visiblelight

Example 5

A pipe having an inner diameter of 38 mm and a length of 10 m wasarranged so as to have an upward slope. A snake pump equipped with ahopper was connected to the inlet of the pipe. In the hopper of thesnake pump, a 44 wt % aqueous sodium hydroxide solution of 40° C. wassupplied at a rate of 1,800 L/hr from a tank. At the same time, 4-mmsquare pulp chips produced by wood and having a solid concentration of93 wt % were introduced at a rate of 50 kg/hr. The rotational speed ofthe snake pump was regulated so that the liquid level inside of thehopper became a predetermined level. The linear speed in the pipe was0.44 m/s, the moving speed of the pulp was 0.44 m/s, and retention timewas 23 seconds. A screw discharge type centrifugal dehydrator wasinstalled as a drainer at the outlet of the pipe type contactor. Amixture of the pulp chips and the sodium hydroxide solution dischargedfrom the pipe type contactor was drained continuously at a centrifugaleffect of 600. The liquid thus separated was introduced into the tankand recycled for the contact with the pulp. A 49 wt % aqueous sodiumhydroxide solution was continuously fed to the tank so as to maintainthe solution in the tank at a constant level. The concentration of thesolution in the tank was maintained at 44 wt %. The weight ratio of thealkali metal hydroxide contained by the alkali cellulose thus obtainedto the solid portion contained by the pulp, was determined by thetitration method, was 1.00.

In a pressure-resistant reactor was placed 5.5 kg in terms of cellulosecontent of the alkali cellulose thus obtained. After vacuuming, 9 kg ofmethyl chloride and 1.4 kg of propylene oxide were added thereto for areaction. The reaction product was then washed, dried and pulverized toyield hydroxypropylmethyl cellulose. The degree of substitution of thecellulose ether, and the viscosity at 20° C. of a 2 wt % aqueoussolution of the cellulose ether and the light transmittance at 20° C. ofa 2 wt % aqueous solution thereof were measured and the results areshown in Table 1. The light transmittance at 20° C. of the 2 wt %aqueous solution thereof was measured using a photoelectric calorimeter“PC-50”, a cell length of 20 mm and visible light.

Example 6

A pipe having an inner diameter of 38 mm and a length of 10 m wasarranged so as to have an upward slope. A snake pump equipped with ahopper was connected to the inlet of the pipe. In the hopper of thesnake pump, a 44 wt % aqueous sodium hydroxide solution of 40° C. wassupplied at a rate of 1,800 L/hr. At the same time, 4-mm square pulpchips produced by wood and having a solid concentration of 93 wt % wereintroduced at a rate of 50 kg/hr. The rotational speed of the snake pumpwas regulated so that the liquid level inside the hopper becameconstant. The linear speed in the pipe was 0.44 m/s, the moving speed ofthe pulp was 0.44 m/s, and retention time was 23 seconds. A screwdischarge type centrifugal dehydrator was installed at the outlet of thepipe type contactor as a drainer. A mixture of the pulp chips and thesodium hydroxide solution discharged from the pipe type contactor wasdrained continuously at a centrifugal effect of 600. The liquid thusseparated was introduced into the tank and by a pump, it was sent to adecanter which was under operation at a centrifugal effect of 2500. Inthe decanter, a fine solid was collected. The fine solid thus recoveredwas mixed with the alkali cellulose. The liquid which had passed throughthe decanter was returned to the tank and recycled for the contact withthe pulp. A 49 wt % aqueous sodium hydroxide solution was continuouslyfed to the tank so as to maintain the solution in the tank at a constantlevel. The concentration of the solution in the tank was maintained at44 wt %. The weight ratio of the alkali metal hydroxide contained by thealkali cellulose thus obtained to the solid portion contained by thepulp, determined by the titration method, was 1.00.

In a pressure-resistant reactor was charged 5.5 kg in terms of cellulosecontent of the alkali cellulose thus obtained. After vacuuming, 9 kg ofmethyl chloride and 1.4 kg of propylene oxide were added thereto for areaction. The reaction product was then washed, dried and pulverized toyield hydroxypropylmethyl cellulose. The degree of substitution of thecellulose ether, and the viscosity at 20° C. of a 2 wt % aqueoussolution of the cellulose ether and the light transmittance at 20° C. ofa 2 wt % aqueous solution thereof are shown in Table 1. The lighttransmittance at 20° C. of the 2 wt % aqueous solution thereof wasmeasured using a photoelectric calorimeter “PC-50”, a cell length of 20mm and visible light.

Example 7

In a similar manner to Example 6 except that the temperature of the 44wt % aqueous sodium hydroxide solution was decreased to 20° C. and thecentrifugal effect of the screw discharge type centrifugal dehydratorwas raised to 1000, alkali cellulose was obtained. The concentration ofthe aqueous sodium hydroxide solution in the tank was kept at 44 wt %.The weight ratio of the alkali metal hydroxide contained by the alkalicellulose thus obtained to the solid portion contained by the pulp,which was determined by the titration method, was 0.60.

In a similar manner to Example 6 except that 6.5 kg of methyl chlorideand 1.2 kg of propylene oxide were added, cellulose ether was prepared.The viscosity at 20° C. and the light transmittance at 20° C. of a 2 wt% aqueous solution of the resulting cellulose ether are shown in Table1.

Example 8

In a similar manner to Example 6 except that the feed rate of the 44 wt% aqueous sodium hydroxide solution was changed to 1,300 L/hr, thecentrifugal effect of the screw discharge type centrifugal separator waschanged to 300, the linear velocity inside the pipe was changed to 0.32m/s, the moving rate of the pulp was changed to 0.32 m/s, and theretention time was changed to 31 seconds, alkali cellulose was prepared.The concentration of the aqueous sodium hydroxide solution in the tankwas kept at 44 wt %. The weight ratio of the alkali metal hydroxidecontained by the alkali cellulose thus obtained to the solid portioncontained by the pulp, which was determined by the titration method, was1.25.

In a similar manner to Example 6 except that 11 kg of methyl chlorideand 2.7 kg of propylene oxide were added, cellulose ether was prepared.The viscosity at 20° C. and the light transmittance at 20° C. of a 2 wt% aqueous solution of the resulting cellulose ether are shown in Table1.

Example 9

In a similar manner to Example 6 except that the feed rate of the 44 wt% aqueous sodium hydroxide solution was changed to 1,300 L/hr, thelinear velocity inside the pipe was changed to 0.32 m/s, the moving rateof the pulp is changed to 0.32 m/s, and the retention time was changedto 31 seconds, alkali cellulose was obtained. The centrifugal effect ofthe screw discharge type centrifugal separator was remained at 600. Theconcentration of the aqueous sodium hydroxide solution in the tank was46 wt %. The weight ratio of the alkali metal hydroxide contained by thealkali cellulose thus obtained to the solid portion contained by thepulp, which was determined by the titration method, was 1.25.

In a similar manner to Example 6 except that 11 kg of methyl chlorideand 2.7 kg of propylene oxide were added, cellulose ether was prepared.The viscosity at 20° C. of a 2 wt % aqueous solution of the celluloseether and the light transmittance at 20° C. of a 2 wt % aqueous solutionthereof are shown in Table 1. TABLE 1 Substitution degree Aqueous 2 wt %solution of cellulose ether of cellulose ether Methoxyl HydroxypropoxylViscosity Transmittance group (DS) group (MS) (mPa.s) at 20° C. (%)Example 5 1.80 0.15  9990 96.5 Example 6 1.80 0.15 10050 96.5 Example 71.40 0.20 10030 93.0 Example 8 1.90 0.25 10020 98.5 Example 9 1.90 0.25 9990 97.0

1. A method for preparing alkali cellulose, comprising the steps of:bringing pulp into continuous contact with an alkali metal hydroxidesolution in a pipe type contactor to generate a contact mixture; anddraining the contact mixture.
 2. The method for preparing alkalicellulose according to claim 1, wherein a weight ratio of alkali metalhydroxide contained by a cake obtained in the draining step to a solidportion contained by the pulp is 0.3 to 1.5.
 3. The method for preparingalkali cellulose according to claim 2, wherein a weight ratio of alkalimetal hydroxide contained by a cake obtained in the draining step to asolid portion contained by the pulp is controlled by one or more ofchanging a linear velocity of the alkali metal hydroxide solution andchanging a contact zone in a pipe of the pipe type contactor.
 4. Themethod for preparing alkali cellulose according to claim 1, wherein aweight ratio of alkali metal hydroxide contained by a cake obtained inthe draining step to a solid portion contained by the pulp is controlledby one or more of changing a linear velocity of the alkali metalhydroxide solution and changing a contact zone in a pipe of the pipetype contactor.
 5. A method for preparing a cellulose ether, comprisingthe steps of: bringing pulp into continuous contact with an alkali metalhydroxide solution in a pipe type contactor to generate a contactmixture; draining the contact mixture; and etherifying the alkalicellulose prepared in the draining step with an etherifying agent toproduce cellulose ether.
 6. The method for preparing a cellulose etheraccording to claim 5, wherein a weight ratio of alkali metal hydroxidecontained by a cake obtained in the draining step to a solid portioncontained by the pulp is 0.3 to 1.5.
 7. The method for preparing acellulose ether according to claim 6, wherein a weight ratio of alkalimetal hydroxide contained by a cake obtained in the draining step to asolid portion contained by the pulp is controlled by one or more ofchanging a linear velocity of the alkali metal hydroxide solution andchanging a contact zone in a pipe of the pipe type contactor.
 8. Themethod for preparing a cellulose ether according to claim 5, wherein aweight ratio of alkali metal hydroxide contained by a cake obtained inthe draining step to a solid portion contained by the pulp is controlledby one or more of changing a linear velocity of the alkali metalhydroxide solution and changing a contact zone in a pipe of the pipetype contactor.
 9. An apparatus for preparing alkali cellulose,comprising: (a) a pipe type contactor comprising (i) at least one inletport at a first end for introducing pulp and an alkali metal hydroxidesolution, and (ii) at least one outlet port at a second end fordischarging a contact mixture, wherein the pulp and the alkali metalhydroxide solution can be moved from the first end to the second endwhile bringing the pulp and the alkali metal hydroxide solution intocontact with each other to generate the contact mixture; and (b) adrainer for separating a cake from the contact mixture discharged fromthe outlet port.